The Buy Box is always centered around whoever has the best offer for the customer, which is usually the lowest priced FBA seller. So, the buy box will rotate to the next best offer. There may be lower priced sellers that get skipped because they are backordered, and Amazon likes to prioritize people who are in stock and who sell FBA. Most likely, it will just go to the next in stock FBA seller at the next lowest price. It could potentially go to an FBM seller, but they would have to have a substantially lower price, have an amazing record as an FBM seller or be closer in location to the customer ordering.
Who wins the buy box once the seller on it sells out?